SummaryThe resistance of tomato (Lycopersicon esculentum) to the pathogenic fungus Cladosporium fulvum complies with the gene-for-gene concept. Host resistance is based on speci®c recognition of extracellular fungal proteins, resulting in a hypersensitive response (HR). Five proteins secreted by C. fulvum were puri®ed and the encoding cDNA clone was obtained from two novel ones among them. Various tomato breeding lines and accessions of Lycopersicon pimpinellifolium were tested for their recognitional speci®city by injection of the puri®ed proteins or potato virus X-based expression of the cDNA. We found that HR-associated recognition of one or more of these proteins, in addition to recognition of the racespeci®c elicitors AVR4 and AVR9 of C. fulvum, occurs among Lycopersicon species. Studies on the inheritance of this recognition con®rmed that single dominant genes are involved. Furthermore, one of the extracellular proteins of C. fulvum is speci®cally recognized by Nicotiana paniculata, which is not a host for C. fulvum. These results indicate that plants have a highly effective surveillance system for the presence of`foreign' proteins, which, together with the high mutation rate of pathogens, can explain the complex gene-for-gene relationships frequently observed in pathosystems.

Nitrogen plays an essential role in the nutrient relationship between plants and pathogens. Some studies report that the nitrogen-mobilizing plant metabolism that occurs during abiotic and biotic stress could be a 'slash-and-burn' defence strategy. In order to study nitrogen recycling and mobilization in host plants during pathogen attack and invasion, the Colletotrichum lindemuthianum/Phaseolus vulgaris interaction was used as a model. C. lindemuthianum is a hemibiotroph that causes anthracnose disease on P. vulgaris. Non-pathogenic mutants and the pathogenic wild-type strain were used to compare their effects on plant metabolism. The deleterious effects of infection were monitored by measuring changes in chlorophyll, protein, and amino acid concentrations. It was shown that amino acid composition changed depending on the plant-fungus interaction and that glutamine accumulated mainly in the leaves infected by the pathogenic strain. Glutamine accumulation correlated with the accumulation of cytosolic glutamine synthetase (GS1 alpha) mRNA. The most striking result was that the GS1 alpha gene was induced in all the fungus-infected leaves, independent of the strain used for inoculation, and that GS1 alpha expression paralleled the PAL3 and CHS defence gene expression. It is concluded that a role of GS1 alpha in plant defence has to be considered.

The interaction between tomato and its fungal pathogen Cladosporium fulvum complies with the gene-forgene system, in which specific recognition of fungal proteins by plant genotypes with matching resistance genes results in host resistance. Two proteins, ECP1 and ECP2, secreted by C. fulvum during infection, are required for full virulence of the fungus on tomato. We chose the most important virulence factor, ECP2, for a targeted search for hypersensitive response (HR)-based resistance among a collection of tomato genotypes. By screening with recombinant potato virus X that expresses the Ecp2 gene, we identified four lines that respond with HR toward ECP2. The capacity to recognize ECP2 and induce HR is sufficient to confer resistance in tomato against C. fulvum producing ECP2. Resistance is based on a single dominant gene, which we have designated Cf-ECP2, for resistance to C. fulvum through recognition of ECP2. Accordingly, an Ecp2-minus strain created by gene replacement is pathogenic on Cf-ECP2 plants. However, due to lack of ECP2 the mutant strain is only weakly virulent. All strains of a worldwide collection of C. fulvum strains that were tested were found to produce a HR-inducing ECP2 protein. Because the Cf-ECP2 gene operates through recognition of an important virulence factor, we expect it will confer durable resistance against C. fulvum. A similar targeted approach should allow the discovery of new valuable resistance genes in other pathosystems.

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